This present invention relates to display devices, such as liquid crystal displays (LCDs) s.
In general, an LCD includes two display panels having pixel electrodes and a common electrode, and a layer of a liquid crystal material having dielectric anisotropy interposed therebetween. The pixel electrodes are arranged in a matrix and connected to switching devices, such as thin film transistors (TFTs), which sequentially apply data voltages to them on a row-by-row basis. The common electrode is disposed over the entire surface of the display panel and has a common voltage applied it. The pixel electrode, the common electrode, and the liquid crystal layer interposed therebetween constitute a liquid crystal capacitor. The liquid crystal capacitor, together with the switching element connected thereto, defines a single pixel unit.
The LCDs images by applying an electric field to the liquid crystal layer disposed between the two panels and adjusting the transmittance of light passing through the liquid crystal layer by controlling the strength of the electric field acting on the liquid crystal layer. However, if a one-directional electric field is applied to the liquid crystal layer for a relatively long period of time, image degradation will occur. To prevent this, the polarities of the data voltages with respect to the common voltage are inverted in units of either a frame of pixels, a row of pixels, or a single pixel.
However, since the response speed of the liquid crystal molecules is relatively low, it takes some period of time for a voltage (hereinafter, referred to as a pixel voltage) charged in the liquid crystal capacitor to reach a target voltage, that is, a voltage which produces the desired luminance in the pixel. The time depends on the difference between the target voltage and the voltage to which the liquid crystal capacitor was previously charged. Therefore, where the difference between the target voltage and the previously-charged voltage is large, if only the target voltage is initially applied, the pixel voltage may not reach the full target voltage during the time in which the pixel switching element is turned on.
In order to address this problem, a DCC (dynamic capacitance compensation) scheme has been proposed. The DCC scheme makes use of the fact that the charging speed is proportional to the voltage across the liquid crystal capacitor. The data voltage (actually, the difference between the data voltage and the common voltage, but for convenience of description, the common voltage is assumed here to be 0V) applied to the pixel is designed to be higher than the target voltage so as to shorten the time taken for the pixel voltage to reach the target voltage.
However, the DCC scheme requires frame memories and driving circuits for performing the DCC calculations. The requirement for these elements creates problems in terms of circuit complexity and a concomitant increase in production costs.
In the case of medium-sized or small-sized LCDs, such as mobile phones, a “row inversion” technique is employed, in which the polarities of the data voltages with respect to the common voltage are inverted in units of pixel rows, so as to reduce power consumption. However, because the resolution of medium-sized or small-sized LCDs is gradually increasing, the power consumption problem is also increasing. In particular, when the DCC calculations are performed, the power consumption of the LCD is greatly increased, due to the additional calculation circuits required.
In addition, in the row inversion technique, the range of data voltages for image display is relatively small in comparison with a “dot inversion” technique, in which the polarities of the data voltages with respect to the common voltage are inverted in units of a pixel. Therefore, in a “VA” (vertical alignment) mode LCD, if a threshold voltage for driving the liquid crystal is high, the range of the data voltage used to represent grays for image display is reduced by the value of the threshold voltage. As a result, the desired luminance cannot be obtained.